fix(gemini): implement model fallback system for reliability

src/services/gemini.ts

@@ -13,29 +13,51 @@ export async function generateGemini(prompt: string, systemPrompt: string): Prom
     throw new Error('Gemini API Key not found. Please set VITE_GEMINI_API_KEY in .env or GEMINI_API_KEY in localStorage.');
   }
 
-  const response = await fetch(`https://generativelanguage.googleapis.com/v1beta/models/gemini-pro:generateContent?key=${apiKey}`, {
-    method: 'POST',
-    headers: {
-      'Content-Type': 'application/json'
-    },
-    body: JSON.stringify({
-      contents: [{
-        parts: [{
-          text: `${systemPrompt}\n\nTask: ${prompt}`
-        }]
-      }],
-      generationConfig: {
-        maxOutputTokens: 1000,
-        temperature: 0.7
-      }
-    })
-  });
+  // Model fallback list - based on March 2026 status
+  const modelOptions = [
+    'gemini-2.0-flash',
+    'gemini-1.5-flash',
+    'gemini-pro'
+  ];
 
-  if (!response.ok) {
-    const errorData = await response.json().catch(() => ({}));
-    throw new Error(errorData.error?.message || 'Failed to generate text from Gemini API');
+  let lastError: any = null;
+
+  for (const modelName of modelOptions) {
+    try {
+      console.log(`Trying Gemini model: ${modelName}...`);
+      const response = await fetch(`https://generativelanguage.googleapis.com/v1beta/models/${modelName}:generateContent?key=${apiKey}`, {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json'
+        },
+        body: JSON.stringify({
+          contents: [{
+            parts: [{
+              text: `${systemPrompt}\n\nTask: ${prompt}`
+            }]
+          }],
+          generationConfig: {
+            maxOutputTokens: 1000,
+            temperature: 0.7
+          }
+        })
+      });
+
+      if (!response.ok) {
+        const errorData = await response.json().catch(() => ({}));
+        console.warn(`Model ${modelName} failed:`, errorData);
+        lastError = errorData;
+        continue; // Try next model
+      }
+
+      const data = await response.json();
+      console.log(`Successfully used model: ${modelName}`);
+      return data.candidates[0].content.parts[0].text;
+    } catch (err) {
+      console.error(`Fetch error with ${modelName}:`, err);
+      lastError = err;
+    }
   }
 
-  const data = await response.json();
-  return data.candidates[0].content.parts[0].text;
+  throw new Error(lastError?.error?.message || lastError?.message || 'All Gemini models failed. Check console for details.');
 }